وَبِـ«أَيْنَ» عَنِ الْمَكَانِ
And ayna asks about place.
أَيْنَ زَيْدٌ؟
Where is Zayd?
A reply such as 'at school' identifies the missing location. The question does not ask whether Zayd exists or how he feels; it seeks spatial information.
Distinguish ayna from kayfa: one asks where, the other asks in what state or manner. Both may be applied to the same individual, but they require different sorts of answers.